The present invention relates to a paste composition using thermoplastic resins, and to a protective film and a semiconductor device both obtained with the same.
With recent progress in downsizing and thinning electric parts, the types of resin sealed packages have been changed from printed board-insertion types (such as DIP: Dual in-line Package) to surface mounting types, for example, Chip on Board (COB) and Flip Chip mounting both for mounting bare chips directly on boards, and further to QFP (Quad Flat Package) and BGA (Ball Grid Array). In these fields, not only conventional thermosetting epoxy resin liquid sealing materials but also thermoplastic resin liquid sealing materials have been developed and marketed. Such liquid sealing materials require high reliability because they directly seal IC or LSI chips. The spread of surface mounting has caused the requirement for high resistance to reflow, such as IR reflow, to ensure reliability of semiconductor elements. Upon mounting insertion-type packages on circuit boards, such as mother boards, only leads are locally heated. However, packages of surface mounting types are subjected to IR reflow, vapor phase reflow or solder dipping on mounting, so that they are wholly exposed to high temperatures of 200 to 250 xc2x0 C. Therefore, the water absorbed in the packages during storage evaporates and expands with heat on mounting, and when the generated stress exceeds the breaking strength of sealing resins, cracks run from the inside of the packages toward outside. Further troubles may arise, for example delamination between sealing resins and passivation films, such as silicon nitride films protecting semiconductor circuits, or disconnection of bonding wires, such as gold wires.
A common approach for improving reflow resistance through modification of sealing materials is to vary compositions of fillers or resins to give sealing materials which are less hygroscopic and reduce the moisture absorption of packages during storage, or have higher bonding strength to base materials or the like. It is, however, difficult for this approach to reconcile improved reflow resistance with good workability, and particularly, has failed to give liquid sealing materials having good reflow resistance. For example, the amount of fillers, such as silica, is increased to give less hygroscopic sealing materials. This can give sealing materials having lower linear expansion, decreasing damage to semiconductor elements. The filling rate can be increased to as much as 90% by weight or higher by optimizing the particle size distribution of silica filler, but the increased filling rates cause problems, such as molding failure due to increased viscosities.
Another means for improving reflow resistance is making through holes for exhausting the water evaporated in semiconductor packages during mounting with heat. For example, as to molding-type sealing materials, Japanese Patent Application Unexamined Publication No. 9-219471 (1997) proposes to drill through holes, and Japanese Patent Application Unexamined Publication No. 9-8179 (1997) proposes to selectively seal the reverse face of semiconductor elements with porous resins. These methods, however, sometimes allow water to enter adversely into semiconductor elements, and cannot ensure sufficient moisture resistance, which is no less important than reflow resistance. Another drawback to the method of selective sealing the back faces of semiconductor elements with porous resins is low productivity because the method needs a step of previously placing porous resin sheets in molds. Further, the method is hardly applicable to semiconductor packages which are difficult to seal by transfer molding using molds, such as cavity-type BGA and xcexc BGA.
Another known method is coating electric or electronic parts, such us Hybrid IC, with porous insulating paints (Japanese Patent Application Examined Publication No. 5-55197 (1993)), which, however, does not aim for improvement in reflow resistance but rather needs secondary moisture proof processing with impregnating materials, such as wax, to prevent the penetration of water.
This invention is made to solve the above-described problems, and an object of this invention is to provide a paste composition for sealing semiconductor having both good reflow resistance and moisture resistance reliability, a protective film using the same and a semiconductor device excelling in reflow resistance and moisture resistance reliability.
Upon various studies for solving the above-described problems, the inventors of this invention have found that semiconductor elements having both good reflow resistance and moisture resistance reliability can be obtained by preparing a paste composition wherein inorganic fillers are dispersed in a solution of resins dissolved in organic solvents, applying it to chips, such as IC or LSI, and drying. It seems to be the reason for this that, in the coating film of the cured resin composition, the voids remaining between the fillers and the fine voids through which evaporated organic solvents escaped remain inside the coating film, and a layer of low water vapor permeability densely filled with the resins is formed on the surface of the coating film.
This invention provides a paste composition, comprising as essential ingredients (A) a thermoplastic resin, (B) an epoxy resin, (C) a coupling agent, (D) a powdery inorganic filler, (E) a powder having rubber elasticity and (F) an organic solvent and which, when applied and dried, gives a coating film having a void content of 3% by volume or higher and a water vapor permeability as measured at 40xc2x0 C. and 90%RH of 500 g/m2xc2x724 h or less.
In this invention, the thermoplastic resin (A) is preferably a polyamide-silicone copolymer or a polyamideimide-silicone copolymer, which is obtainable by polycondensing an aromatic dicarboxylic acid, an aromatic tricarboxylic acid or a reactive acid derivative thereof with a diamine containing as an essential ingredient a diaminosilicone.
In this invention, the epoxy resin (B) is preferably an alicyclic epoxy resin.
In this invention, the coupling agent (C) is preferably one or more coupling agents selected from a silane coupling agent, a titanate coupling agent and an aluminate coupling agent.
In this invention, the powder (E) having rubber elasticity is preferably silicon rubber powder.
This invention further provides a protective film which is formed by applying the paste composition to a surface of a semiconductor part and drying it and has a void content of 3% by volume or higher and a water vapor permeability as measured at 40xc2x0 C. and 90%RH of 500 g/m2xc2x724 h or less.
This invention further provides a semiconductor device having the protective film.
In the semiconductor device of this invention, the protective film is preferably a sealing material sealing a chip in the semiconductor device.
In the semiconductor device of this invention, voids of 1 to 20 xcexcm diameter in the sealing material preferably have a total volume of 1 to 100% by volume of a total volume of voids of 0.0032 to 100 xcexcm diameter in the sealing material.
In the semiconductor device of the above-described embodiment, it is preferable that the chip is mounted on a chip-supporting board which comprises an inner connection part to which a terminal of the chip is connected, an outer connection part for connection with outside of the semiconductor device, and a interconnecting part connecting the inner connection part with the outer connection part. For example, it is preferable that the inner connection part is a interconnecting pattern formed on one side of an insulating base material, the outer connection part comprises a plurality of electrode parts formed on the other side of the insulating base material and connected to the interconnecting pattern by the interconnecting part, the chip is bonded to the side of the insulating base material bearing the interconnecting pattern by an adhesive for bonding semiconductor element, and the terminal of the chip and the interconnecting pattern are connected to each other by a gold wire.